Elastic sealing apparatus



s w L M A UP .P. 7MB m RT. m Ww C WI .m m E Original Filed March 24,1950 United States Patent 2,757,423 ELASTIC SEALING APPARATUS John W.Wurtz, San Gabriel, and Fred A. Greenwald, South Gate, Calih, assignorsto National Motor Bear ing Co., Inc, Redwood City, Calif., a corporationof California Original application March 24, 1950, Serial No. 151,766,now Patent No. 2,686,747, dated August 17, 1954. gigiged and thisapplication March 11, 1952, Serial No.

1 Claim. or. -49

This invention relates to an improvement in sealing materials. Moreparticularly, it relates to the manufacture of sealing materials forpreventing and controlling the flow of fluids over a wide range oftemperature; still more particularly, the invention relates to thesealing of structures such as the bodies of vehicles and the cabins ofairplanes against egress or ingress of air where the ma terial issubjected to extremely low temperatures at certain times at whichordinary sealing materials become embrittled and hardened. Thisapplication is a division of application Serial No. 151,766, filed March24, 1950, which issued August 17, 1954 as Patent No. 2,686,747.

In the sealing of airplane cabins, it is desirable that the sealingmaterial maintain its elasticity at temperatures of the order of 60 F.to 80 F. when employed at high altitudes. Such material must alsowithstand temperatures of 100 F. to 150 F. at low altitudes and at otherseasons and may sometimes be subjected to temperatures of 200 F. to 300F. It has heretofore been difficult to obtain satisfactory sealing ofsuch structures with rubber and similar materials commonly in use.

We have provided a satisfactory sealing product for this service from anelastic organic silicone compound formed into a cellular structure orsponge. For this purpose we prefer to use the rubber-like polymethylalkylsiloxanes retained in a suitable jacketing material for support andto prevent abrasion of the relatively weak polysiloxane compound.Previous attempts to use this material in solid form for sealingpurposes have been unsuccessful owing to the low resilience of thepolysiloxanes, resulting in serious deformation under prolonged stress.We have solved this problem by incorporating inert gases in thepolysiloxane in a cellular structure as hereinafter described.

The invention is described in a drawing which shows in Figure l partlyin perspective and in cross section a typical form of seal employed forsealing the openings in airplane cabins operating at high altitudesunder pressurized conditions. Figure 2 is a cross-section of a similarseal product in modified form.

In the preparation of our sealing material we may employ polyethylsiloxane, polyisopropyl siloxane, etc., but we prefer to employpolydimethyl siloxane which is an elastomer marketed in various gradesunder the name of Silastic. To this material we add about /2 to 2% of anexpanding agent which may suitably be a thermally sensitive organicdiazo or nitroso compound; for example, we may add about 1% ofdiazoaminobenzene. Thorough mixing is obtained by milling on squeezerolls or by other suitable mixing device. The temperature of mixingshould not be permitted to rise above ordinary temperatures; e. g. 100F. to 150 F. Instead of the nitrogen compound we may use other expandingagents capable of disengaging gases when heated. Sodium bicarbonate, forexample, can be employed. In some cases where a high rate of heating isemployed in the expanding step, to be hereinafter described, thedissolved gases in the Silastic are suflicient to effect the desiredexpansion without addition of any expanding agent.

A curing agent for the polysiloxane is also required. For this purposevarious oxidizing agents are used, preferably organic peroxides such asbenzoyl peroxide, acetyl peroxide, diethyl peroxy dicarbonate, etc. inamounts of about 0.1 to 5 g.

After uniformly incorporating the expanding agent, the Silasticcombination is next formed into approximately the desired shape, forexample, by extruding through a die. Thus we may form it into rods ortubes. The resulting extruded product is next heated to activate theexpanding agent, thereby forming the polysiloxane into a sort of spongeand greatly increasing: its compressibility. It is usually sufficient toheat to a temperature of about 250 F. to 550 F. When employingdiazoamino benzene, a temperature of 400 F. for a period of ten minutesis usually suliicient. However, different grades and different lots ofthe polysiloxane material require somewhat different heating times andtemperatures which can be readily determined in advance by experiment.It is desirable to control the initial heating-both with respect totemperature attained and rate-to produce a cell structure in which thecell diameter is about Ms inch, with a cell wall thickness of about .002inch.

After the initial heating, the polysiloxane rubber product now in theform of a soft sponge is covered to provide resistance to abrasion andto increase its strength and general applicability. For this purpose itis preferred to employ a fabric comprised of glass fiber or nylonbecause of the great strength and durability of these materials andtheir stability at the required curing temperatures. It is desirable tocement the cover to the polysiloxane sponge by means of a polysiloxanecement which is suitably a dispersion of partially polymerized dimethylsiloxane in a solvent such as xylene or tetrachlor ethylene. If desired,the covering fabric may be completely impregnated with the cement toincrease its durability. Other cements such a Vinylite and rubber,suitably in the form of an emulsion or latex, may be employed but areless desirable because of their lower chemical stability and greatersusceptibility to hardening in use at low temperatures.

The drawing shows a section of a sealing material pre pared in the formof a cylinder or rope with a web for convenience in fastening. Thespongy polysiloxane body 10, is surrounded by a flexible sheathing 11which is brought together in a seam at 12 and formed into a web 13. Alayer of cement 14 attaches the spongy polysiloxane core to the coverwhich is usually fabric.

After the fabric sheath has been applied to the poly siloxane sponge,the product is subjected to heat in a curing oven usually for about 4 to12 hours at a temperature of about 300 F. to 500 F. for the purpose ofcuring the polysiloxane sponge and preventing it from collapsing underlong sustained pressure and deformation. The curing treatment alsoserves to set the Silastic cement employed in attaching the sheath tothe core.

In another modification of our invention, we inject the unexpandedcomposition of polyalkylsiloxane rubber and expander into a suitablesheath before the initial expanding heat treatment, thereby expanding itdirectly in place. When carried out in this way, the process permitsexpanding and curing the material in a single heating operation whichusually extends for a period of 4 to 12 hours at a temperature of from300 F. to 550 F. It is sometimes advantageous when operating in this wayto effect the initial expansion at a higher temperature, e. g. 450 F.and then lower the temperature to 350 F. and hold at that temperaturefor the remainder of the curing time. According to this modification ofthe process, the polysiloxane composition containing the expander, i. e.

3 diamino-azobenzene, can be extruded directly into the sheath in anamount suflicient to slightly more than fill the volume of the sheath onexpansion. The sheath may be initially coated on the inside with asuitable cement to provide a firm bond between the sheath fabric and thepolysiloxane sponge. Various bonding agents as heretofore described canbe employed. However, the use of a bonding agent is not obligatory whenoperating in this manner inasmuch as there is a tendency for thepolysiloxane to become self bonded to the sheath fabric on expansion.Still other types of flexible sheath material use no fabrics at all. Forexample, we may employ for this purpose an elastic tube of neoprene(polychloroprene) or Buna rubber. In this case it is necessary to insertthe cured polysiloxane sponge into the sheath, generally after coatingwith suitable cement; e. g. rubber cement or polyvinyl acetate cement tofacilitate assembly. If desired, the rubber may be initially expanded bytreating with a light hydrocarbon such as benzene or hexane. Onevaporation of the hydrocarbon after assembly, the sheath can be shrunkonto the polysiloxane core and the two parts may then be bonded byheating in an oven or autoclave to a temperature sufficient to softenthe thermoplastic cement previously applied to the intervening surfaces.In another modification of our product, a sheath material ofpolysiloxane film or tubing may be applied to the surface of thepolysiloxane rubber sponge; for example, by cementing and wrapping or byintrusion as just hereinabove described. Formation of the sponge withina dense polysiloxane tubular sheath may also be effected by expanding insitu.

It is usually desirable to incorporate one or more inorganic modifyingagents or extenders in the mixture of polysiloxane and expanding agent.These inorganic modifying agents generally increase the hardness of theresulting cellular polysiloxane product and reduce excessive flexibilitywhere that is desirable. For this purpose we may employ zinc oxide,alumina gel, bauxite, fullers earth, any of the numerous forms of ironoxide or ferric oxide such as ochre, rouge, umber, titanium oxide andsilica gel. When employing silica gel, we prefer to use the alcogelbecause of its extremely low density. All of the above inorganicextenders are employed in a finely divided form, generally having aparticle size in the range of about 1 to 30 microns. The amount ofinorganic extender may vary from about 5% to 40%, usually about to 25%.

In addition to those materials hereinabove described for sheathing thecellular polysiloxane we may employ woven metal fabric such as brass,copper or aluminum wire cloth or a flexible paper cover such as crepepaper. Glass fibre or wire cloth impregnated with elastic polymerizedcarbofluorides (Teflon), polyvinyl resins, neoprene or other elastomercan also be used. We may also stiffen or reinforce the web 13 as shownin Figure 2 of the drawing by insertion of a thin metal strip or wire15. A strip of thin sheet brass, copper, aluminum or zinc may beemployed and, if desired, it may be perforated to facilitate fastening.The use of a deformable soft metal strip or wire in the web eithercompletely enclosed or cemented to one face thereof permits the shapingof the sealing material to a form which will be retained duringhandling.

If desired We may soften the sponge polysiloxane rubber material bysqueezing, for example through rollers, after the initial cure andbefore the final hardening treatment. We prefer to crush the spongematerial sufficient to break a major part of the cell structure, thenapply the sheath and proceed with the final heat treatment. The softenedproduct suffers less distortion in the final heating owing to escape ofgases from the ruptured cells, and the bond to the sheath is not damagedby expansion.

The product of the invention is characterized by the unique property ofmaintaining a high degree of com pressibility and elasticity over a longperiod of time and over a wide range of temperatures from about F. toupwards of +400 F. We have found that the gases trapped in the cells ofpolysiloxane are retained much more persistently than in the case ofother elastic materials attempted to be used for this purpose. Webelieve that the outstanding advantage of our product is due partiallyto the low rate of diffusion of gases such as nitrogen, oxygen, air andcarbon dioxide through the cellular structure of our expandedpolysiloxane. We believe further that the incorporation of the expandedpolysiloxane material within a sheath resistant to the passage of gasessuch as cemented or impregnated glass or nylon fabric or within a sheathor rubber, Silastic, neoprene, Buna N, Buna S or other elastomer servesto still further maintain the compressibility of our product and preventcollapse over a long period of time when employed as a sealing agentunder conditions where the material is greatly deformed by compressiveforces.

Having thus described our invention, what we claim is:

A Weatherstrip for sealing closures in airplane cabins and the like,comprising an elastomeric polydimethyl siloxane body containing between5% and 40% by weight of inorganic extender, and having a cellularstructure with a major portion of the cell Walls ruptured to providepassage for gas therethrough, the cells having a diameter in the orderof about /6 with a cell wall thickness in the order of about 0.002; anda sheath of cloth made from glass fibers encasing said body and bondedto said body by cured elastomeric polydimethyl siloxane cement, wherebysaid Weatherstrip maintains its flexibility and elasticity over a rangeof temperatures between about 120 F. and about 400 F., and thecombination maintains its compressibility and resistance to collapseunder severe conditions of temperature and deformation.

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